Cylindrical specimens of cast polycrystalline nickel base superalloy Inconel 738LC were cyclically strained under total strain control at 23 and 800 °C to fracture. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves were obtained at both temperatures. Surface relief was studied in specimens fatigued to failure using scanning electron microscopy. Cyclic hardening/softening behaviour depends both on temperature and strain amplitude. Low amplitude straining was characterized by saturation of the stress amplitude. In high amplitude straining a pronounced hardening was found which was followed by saturation at room temperature and by cyclic softening at 800 °C. The cyclic stress-strain curves can be fitted by power law. They are shifted to lower stresses with increasing temperature. Fatigue life curves can be approximated by the Manson- Coffin and Basquin laws. The Manson-Coffin and Basquin curves are shifted to lower lives with increasing temperature. Slip markings were detected on specimen surface at all test temperatures. When temperature grows the density of slip markings is reduced.

In this paper, the minimum temperature and phase shift effects on the thermo–mechanical fatigue (TMF) behavior of Inconel 713LC are investigated. TMF tests were performed under 0° (in-phase-IP) and +180° (out-of-phase-OP) phase shifts between mechanical strain and temperature. Cylindrical specimens were cycled at constant mechanical strain amplitude with a strain ratio of R_ε = −1. Tests were performed with temperature ranges of 300–900 °C and 500–900 °C. The heating and cooling rate was 5 °C/s. Fatigue hardening/softening curves and fatigue life data were assessed. Results show that out-of-phase loading was less damaging than in-phase loading. Scanning electron microscopy (SEM) examination of metallographic sections indicated that the life-reducing damage mechanism was intergranular cavitation under in-phase loading. Transmission electron microscopy (TEM) revealed honeycomb structures for IP loading. The plastic strain localization into persistent slip bands was typical for OP loading. For out-of-phase loading, fatigue damage appeared to be dominant. The increase in the temperature range led to a significant decrease in fatigue life. The reduction of fatigue life was far more pronounced for out-of-phase loading. This can be ascribed to the accelerated crack propagation at high tensile stress under out-of-phase loading as well as the amount of accommodated plastic strain deformation. Based on the SEM scrutiny of metallographic sections and TEM observations of dislocation arrangement, the prevailing damage mechanisms were documented and the lifetime behavior was accordingly discussed.